Winners don’t punish

Abstract

Many theories have been offered to explain the evolution of cooperation in humans. One proposal is that costly punishment can promote cooperation. Everyone benefits on average, the theory goes, despite the cost to those doing the punishing. But most of our interactions are repeated, and in such cases punishment can lead to retaliation. Using a variant of the 'Prisoner's Dilemma' game, Dreber et al. find that punishment increases the frequency of cooperation, but not the average payoff. Costly punishments confer no overall advantage to the group. And players who end up with the highest total payoff ('winners') tend not to use punishment, while those with the lowest payoff ('losers') punish most frequently. It seems that costly punishment may not have evolved to promote cooperation, but for some other purpose. An experimental economics approach finds that punishment increases the frequency of cooperation, but not the average payoff. Thus, the option of costly punishment does not confer an advantage to the group. Moreover, players who end up with the highest total payoff ('winners') do not use punishment, whereas players who end up with the lowest payoff ('losers') use punishment most frequently. A key aspect of human behaviour is cooperation1,2,3,4,5,6,7. We tend to help others even if costs are involved. We are more likely to help when the costs are small and the benefits for the other person significant. Cooperation leads to a tension between what is best for the individual and what is best for the group. A group does better if everyone cooperates, but each individual is tempted to defect. Recently there has been much interest in exploring the effect of costly punishment on human cooperation8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23. Costly punishment means paying a cost for another individual to incur a cost. It has been suggested that costly punishment promotes cooperation even in non-repeated games and without any possibility of reputation effects10. But most of our interactions are repeated and reputation is always at stake. Thus, if costly punishment is important in promoting cooperation, it must do so in a repeated setting. We have performed experiments in which, in each round of a repeated game, people choose between cooperation, defection and costly punishment. In control experiments, people could only cooperate or defect. Here we show that the option of costly punishment increases the amount of cooperation but not the average payoff of the group. Furthermore, there is a strong negative correlation between total payoff and use of costly punishment. Those people who gain the highest total payoff tend not to use costly punishment: winners don’t punish. This suggests that costly punishment behaviour is maladaptive in cooperation games and might have evolved for other reasons.